Method in the manufacture of wood pulp from chips in grinding apparatus in two stages



June 11, 1968 A. J. A. ASPLUND ET AL 3,388,037

METHOD IN THE MANUFACTURE OF WOOD PULP FROM CHIPS IN GRINDING APPARATUS IN TWO STAGES Filed May 28, 1964 Unite I ABSTRACT OF THE DISCLGSURE A method of manufacturing wood pulp for paper production which consists in heating wood chips under steam pressure at high temperature exceeding 100 C. to minimize or to avoid discoloration of the chips; carrying out defibration in two steps between grinding members, such as first in a defibrator and then in a refiner. The defibration in the first step is effected under high pressure and high temperature and in the second step under high pressure and lower temperature. Chemicals in solution or suspension are continously applied to the chips during the first or during both steps prior to the passage of the chips between the grinding members without noticeable loss of organic substance and the chemicals have a bleaching effect during defibration, and then the pulp suspension is cooled by a rapid reduction of the pressure.

This invention relates to the manufacture of cellulose pulp.

More particularly this invention relates to the manufacture of cellulose pulp by defibration of wood chips in grinding apparatus.

It is known to defibrate Wood chips in grinding apparatus of the disc type, for example, under steam pressure at temperatures exceeding 100 C., such as within the range of 140 to 150 C., consid rably less power being required for the defibration process than when the process is conducted at temperatures below 100 C. The term chips is in the following description understood to include also other forms or kinds of disintegrated, lignocellulose, containing materials.

The main disadvantage in the defibration or refining of chips under pressure and at a high temperature, in particular within the range between 140 and 220 C., is constituted by decomposition and discoloring of the pulp due to hydrolysis. A short time of heating reduces this drawback, but even if the heating time is chosen so short that the chips are brought to the temperature of defibration immediately at the moment when they enter between the grinding discs, the color of the pulp will not become satisfactory for production of, or intermixture with other constituents of newsprint.

It is known for some time that pulps presenting satisfactory brightness such as 60 to 70 percent G.E., can be produced with high yield by impregnating the chips with sulfite solutions and heating them under steam pressure prior to a defibration. In those cases where some kinds of deciduous wood such as birch or beech are intended to be used, the impregnation way has proved to be best. This applies also to those cases where high mechanical strength values are desired.

If, however, the defibration of conifers such as spruce and pine or some other than the deciduous wood types mentioned above, such as aspen, for example, are in consideration for the production of mechanical pulp possessing high brightness and with high yields such as be- States Patent tween and percent, it has proved possible completely to dispense with the impregnation and instead to feed the sulfite solution to the chips immediately prior to the entering of the chips between the grinding discs. The sulfite solution acts mainly as a bleaching agent and the dissolving out of organic substance remains at low values. The type of pulp to which the invention primarily is to be applied may be defined as a pulp capable of replacing, or becoming completed by, pulp produced with conventional grinders.

A condition for obtaining excellent brightness and acceptable mechanical strength of the pulp is constituted by the time of heating of the chips including the time for the defibration being short. When operating within the tem perature range between and 220 C. and in particular within the range between and C., the heating time must not exceed 5 minutes and preferably is kept below 2 minutes. The heating time in this connection is calculated from the moment when the chips have reached the predetermined final temperature. For this reason the pulp must be cooled rapidly as soon as it has passed through the grinding discs which most suitably is effected by a rapid pressure reduction and expansion resulting thereof of steam. The chemicals are supplied in a preferably aqueous solution or suspension to the heated chips.

The following description of a plant suitable for carrying out the method of the invention and shown in a partially sectional projection in the accompanying drawing which forms part of this specification, is intended merely for illustration and not for limitation of the invention in order to show properly how the method may be carried out and which advantages it has over the prior art.

The chips to be treated are introduced through a sluice or a so called cell-feeder 10 into a preheater 12. This preheater is supplied with steam through a pipe 14 controlled by a valve 16 provided therein. The preheater may have outlet 18 provided with a control valve 20 for consumed steam and/0r gases escaping from the chips. The chips descend in the preheater and hereunder they are heated rapidly to the predetermined treatment temperature, such as between 160 and 180 C., depending on the temperature and pressure of the steam. Located at the base of the preheater 12 is a screw conveyor 28 driven through a sprocket 24 by a motor (not shown) and adapted to convey the preheated chips through a channel or passage 26 to a defibrator 28 provided with two grinding discs 30, 32 in a manner known per se. The defibrator may be of the type described in the co-pending patent application Ser. No. 237,911 filed Nov. 15, 1962, by Arne Johan Arthur Asplund, now abandoned. The grinding disc 32 is driven by a motor 34. A solution of chemicals, preferably sulfite, is supplied to the chips immediately prior to their entering between the grinding discs 30, 32. This may be eifected through the intake 36 located in the channel 26.

The defibrated pulp is discharged through a conduit 38 in which is provided a sluice or throttle 40. In the defibrator 28 the same pressure is prevailing as in the preheater 12, but by the passage through the sluice 40 a sudden reduction of pressure and corresponding reduction of temperature is provoked. In the embodiment shown a refiner 42 is disposed in direct connection to the defibrator 2% for which reason the conduit 38 opens into the inlet of the refiner. The refiner has grinding discs 44, 46 of which the latter is driven by a motor 48. The refiner may be principally of the same kind as the defibrator 28 but designed so as to bring about a more advanced disintegration of the fibers and/or splitting up of the individual fibers. The temperature in the refiner may be kept below or above 100 C. Here also chemicals are supplied to the pulp immediately prior to its entrance between the grinding discs, for example through a conduit 50 which in the embodiment shown in the drawing opens radially ahead of the grinding interspace between the grinding discs. The feed way for the pulp to said grinding discs after the impregnation with the chemicals will thus be very short. The refiner 42 has a discharge 52 provided with a sluice or valve 54 for continuous conveyance of the pulp, for example into the surrounding free atmosphere or for further treatment.

Also in the defibrator 28 the solution of chemicals may be introduced into the chip mass at a location straight inside the grinding discs. To this end the shaft 23 of the screw conveyor 22 may be made hollow and thereby become adapted to serve as supply channel opening adjacent the grinding discs.

The bleaching effect of the sulfite solution can be increased by an admixture of hydrosulfite or zinc oxide white powder. The subsequent refining of the defibrated pulp may be effected with or without hydrosulfite, free S or free S0 and zinc oxide white being present. When finally bleaching the defibrated pulp with sodium peroxide or hydrogen peroxide or hyprochlorite, diluted alkali solution may be added during the defibrating operation in order to counteract possible hydrolysis. The color of the pulp will become less satisfactory than when using sulfite solution, but instead it becomes easier to perform a final bleach with the bleaching agents mentioned hereinbefore. When using hypochlorite for the :bleach, the pH value of the pulp suspension must be kept at about 9. To avoid heavy brown coloring of the pulp in the defibration step conducted in an alkaline environment, a weak solution of sodium peroxide or an alkali solution to which a peroxide is added, may be used. The alkaline solutions may be constituted by alkalihydroxide and/ or alkali carbonate. In some cases it has shown possible to use suspensions of hydrates or hydroxides and carbonates of alkaline earth metals such as calcium hydroxide and magnesium hydroxide or calcium carbonate and magnesium carbonate. The chemicals when consitituted by several components may be supplied to the grinding apparatus mixed together or each separately.

While one more or less specific embodiment of the invention has been shown and described, it is to be understood that this is for purpose of illustration only and that the invention is not to be limited thereby, but its scope is to be determined by the appended claims.

What we claim is:

1. Method of continuously producing pulp having a yield between 85% and 95% from lignocellulose material which comprises:

(a) heating the material in apreheater by steam at 5 a temperature of between 140-240 C. and corresponding superatmospheric pressure;

(b) passing the thus heated material to a first defibrating zone while maintaining substantially the steam temperature and superatmospheric pressure as in the preheater;

(c) the combined heating and defibration in (a) and (b) being carried out for a period from 2 to 5 minutes;

(d) passing the thus defibrated pulp to a second defibrating zone under reduced pressure and temper- :ature; and

(e) supplying a chemical solution to the pulp before its introduction into at least one of said defibrating zones.

2. The method according to claim 1, characterized by the step of supplying a solution of chemicals consisting of sulfite solutions with or without admixture of hydrosulfite or/ and zinc oxide white powder.

3. The method according to claim 1, characterized by the step of supplying a solution of chemicals consisting of peroxides.

4. The method according to claim 1, characterized by the step of supplying a solution of chemicals consisting of a solution of alkaline hydroxides and/or alkaline carbonate and/or a suspension consisting of hydroxides and/or carbonates of alkaline earth metals and a subsequent final bleaching of the pulp.

References Cited UNITED STATES PATENTS 1,711,706 5/1929 Weiss 162-26 1,870,650 8/1932 Richter 162-85 2,454,532 11/1948 Walter 162- 26 X 2,957,795 10/1960 Stuck 162 28 2,960,161 11/1960 Richter 162-237 3,023,140 2/1962 Textor 16226 X DONALL H. SYLVESTER, Primary Examiner. HOWARD R. CAINE, Examiner. 

